An automatic transmission has been in actual use that has an electromagnetic valve and receives an externally applied electric signal to adjust, by means of an ECU (Electronic Control Unit), such variables concerning gear-ratio change as gear position, hydraulic-pressure level, time constant and timing for example for the gear-ratio change. Using such an ECU, the operating state of the automatic transmission can surely and speedily be changed to various states. Further, the ECU incorporates therein a CPU (Central Processing Unit) and thus can be controlled by a program. Therefore, through changes of the program and various constants, the operating state of the automatic transmission may be set delicately. Then, according to the running state of the vehicle and the load state of the engine, the optimum performance can be derived from the automatic transmission. Here, the running state of the vehicle refers to the vehicle speed, steering operation, the frequency of acceleration/deceleration and the level thereof, and the road surface state for example. The engine load state refers to the rotational speed of the engine, the opening position of the throttle, the press-down degree of the accelerator, the torques of input and output shafts of the engine and automatic transmission for example.
Further, the pressure level of fluid supplied to engagement elements (such as clutch and brake) incorporated in the automatic transmission is delicately adjusted according to the running state of the vehicle and the load state of the engine. By this adjustment, while both of suppression of shift shock and reduction of wear of the engagement elements are achieved, gear ratios can be changed speedily and smoothly.
Such an automatic transmission is configured to connect to an engine via a torque converter or the like and have a transmission mechanism including a plurality of power transmission paths and, for example, automatically change the power transmission paths based on the press-down degree of the accelerator and the vehicle speed. In other words, the automatic transmission is configured to automatically change gear ratios (speed gears). Generally, a vehicle having the automatic transmission is provided with a shift lever to be operated by a driver. According to the operation of the shift lever, the gear position (for example, reverse drive position, neutral position, forward drive position) is set. Within the range of the gear position thus set (usually within the range of the forward drive position), automatic transmission control is performed.
When the vehicle having the automatic transmission as described above is in the condition where the forward drive position is selected while the vehicle stops, a driving force from the engine which is idling is transmitted via the torque converter to the transmission and then transmitted to wheels, resulting in so-called creeping. The creeping is very useful under some predetermined conditions. For example, the vehicle stopping on an uphill slope can be started smoothly. However, when the vehicle should be stopped and kept as it is, the creeping is unnecessary. Thus, the vehicle brake is operated to suppress the creeping force. In other words, the creeping force from the engine is suppressed by the brake, which means that the problem of deterioration in engine fuel economy arises accordingly.
In view of the above, it has been proposed to improve the fuel economy in the following way. In the condition where a forward drive position is selected and the brake pedal is depressed to operate the brake while the accelerator is substantially entirely closed to stop the vehicle, a forward clutch is disengaged while the forward drive position is kept, so that the transmission is in a neutral state which is close to the neutral.
Many techniques have been disclosed including the technique called neutral control as described above as well as a control technique for the state where a vehicle having been stopped is to be started immediately. In particular, a technique is known by which a lockup clutch capable of directly coupling the input and the output of the torque converter is controlled. Specifically, an engaging force of the lockup clutch is fed-back controlled to a predetermined state (slip control) according to a difference between the input pump rotational speed (corresponding to the engine speed) and the output turbine rotational speed. Thus, the slip state of the torque converter is appropriately controlled to prevent vibrations and noise from generating, while improving the starting performance of the vehicle.
Thus, under the sophisticated electronic control, the power transmission ratio between mechanical power transmission by the lockup clutch and power transmission by the torque converter is delicately controlled according to the running state so as to remarkably improve the transmission efficiency. Under this control, an intermediate mode (slip control providing a very small slip to the lockup clutch) is applied extensively to a low-vehicle-speed range so as to expand the lockup region.
Japanese Patent Laying-Open No. 2005-3193 discloses a control apparatus for a lockup clutch of a vehicle, performing slip control for the lockup clutch in returning from neutral control to start the vehicle. The control apparatus for the lockup clutch of the vehicle is a control apparatus for controlling a lockup clutch of a vehicle having a hydraulic power transmission device equipped with the lockup clutch. The hydraulic power transmission device is disposed on the output side of an engine. An automatic transmission is coupled to the output side of the hydraulic power transmission device equipped with the lockup clutch. The control apparatus includes: a neutral control unit releasing a hydraulic frictional engagement device for releasing a power transmission path in the automatic transmission when the vehicle is stopped; an original pressure control unit raising an original pressure of the hydraulic frictional engagement device by a predetermined level during control of the neutral control unit for releasing the power transmission path of the automatic transmission, and gradually reducing the original pressure after the releasing control is finished for returning; and a lockup clutch control unit setting the lockup clutch in a slip state when the vehicle is started. The control hydraulic pressure used for the control of the lockup clutch is produced from the original pressure controlled by the original pressure control unit.
Regarding this control apparatus for the lockup clutch of the vehicle, when the control returns from the neutral control to start the vehicle, the lockup clutch control unit sets the lockup clutch in the slip state. Thus, when the vehicle is to be started, in parallel with transmission of the transmission torque from the engine to the subsequent stage via the hydraulic power transmission device, the transmission torque is also transmitted via the lockup clutch to the subsequent stage. Therefore, as compared with the conventional start with power transmitted by only the hydraulic power transmission device, any increase in rotational speed of the engine when the vehicle is to be started is suppressed. Therefore, good fuel economy is obtained in starting the vehicle (the start of the vehicle as described above is hereinafter referred to as slip start). Further, the original pressure control unit gradually reduces, when the neutral control is finished, the original pressure which is raised by a predetermined pressure during the neutral control by the original pressure control unit. Thus, when the vehicle is to be started immediately after the neutral control, the lockup clutch control unit sets the lockup clutch of the hydraulic power transmission device in the slip state, and a sudden change of the original pressure used for slip control of the lockup clutch can be prevented. Therefore, any disturbance in slip control due to the sudden change of the original pressure is appropriately eliminated.
However, regarding the control apparatus for the lockup clutch of the vehicle as disclosed in Japanese Patent Laying-Open No. 2005-3193, when the accelerator pedal is depressed in returning from the neutral control, the slip start could be done in some cases. In returning from the neutral control, the forward clutch (also referred to as input clutch) having been disengaged is engaged. If the slip start is performed in an initial stage of the forward clutch engagement, the turbine torque would vary to a large extent, resulting in deterioration of the engagement characteristic of the forward clutch. Thus, there arise such problems as occurrence of engagement shock as well as durability.